In the related art, voice calls in a mobile communication system of the third generation partnership project (3GPP) are made using a 3GPP circuit switching (CS) network. In recent years, a Voice over Long Term Evolution (VoLTE) service, which provides a voice call using a 3GPP packet switching (PS) network, has been started.
However, the area where the VoLTE service is available is limited for a while. For this reason, when a user moves out of the VoLTE service area during a voice call using on VoLTE (hereinafter, refer to as VoLTE call), it is necessary to switch this call to a call based on a circuit switching technique according to the related art. As a technique that enables this switching, there is single radio voice call continuity (SRVCC) disclosed in Non-Patent Literature (hereinafter, abbreviated as “NPL”) 1. Hereinafter, a handover operation based on SRVCC will be described with reference to FIGS. 1 and 2.
FIG. 1 is a diagram illustrating a part of a configuration of a 3GPP mobile communication network. The mobile communication network shown in FIG. 1 is configured using an evolved universal terrestrial radio access network (e-UTRAN), an e-UTRAN base station (e-nodeB), a PS network, a CS network, a base station subsystem of the CS network, and an IP multimedia Subsystem (IMS).
Specifically, in FIG. 1, e-UTRAN is a radio access network that is capable of providing the VoLTE service. The PS network provides the VoLTE service and includes a packet data network gateway (P-GW), a serving gateway (S-GW), and a mobility management entity (MME). The CS network includes a mobile switching center (MSC), and a media gateway (MGW). The base station subsystem of the CS network includes a radio network controller (RNC), and nodeB. IMS performs a call control or the like, and includes a call session control function (CSCF), and a service centralization and continuity application server (SCC AS). Note that in FIG. 1 and FIG. 2, MSC and MGW are represented as a single node (MSC/MGW 110), but may be provided as separate nodes.
In FIG. 1, it is assumed that UE 100 and UE 102 that are mobile communication terminals (user equipment) are initially connected to the PS network, respectively (here, a radio access network, a base station and a PS network on the side of UE 102 are not shown). That is, it is assumed that a VoLTE call is made between UE 100 and UE 102. Here, it is assumed that UE 100 is handed over (HO) to the CS network during the call.
Path A, Path B and Path C indicated by solid lines in FIG. 1 represent paths through which speech data passes. Further, reference numerals 200, 202, 204 and 206 indicated by dashed lines in FIG. 1 represent paths through which signals pass in an SRVCC handover process.
FIG. 2 is a sequence chart illustrating an operation of the SRVCC handover process. UE 100 and UE 102 are initially connected to the PS network (e-UTRAN), respectively, and the speech data between UE 100 and UE 102 is transmitted and received through Path A. If UE 100 is distant from a cover area of the e-UTRAN, e-nodeB detects the fact, and exchanges signaling with RNC/nodeB through MME and MSC/MGW 110 (signaling 200 shown in FIG. 1 and step (hereinafter, referred to as “ST”) 200 shown in FIG. 2). In ST200, a data path in the CS network is prepared between nodeB and MSC/MGW 110. If the preparation is finished, a command for handover to UTRAN (CS network) is given to UE 100 from MME through e-nodeB.
At the same time with the process of ST200, MSC/MGW 110 exchanges signaling with UE 102 through CSCF/SCC AS (signaling 202 shown in FIG. 1 and ST202 shown in FIG. 2). Thus, a command is given for switching a transmission/reception destination of speech data of UE 102 from UE 100 to MSC/MGW 110, and Path B is established.
After handover to UTRAN, UE 100 exchanges signaling with MSC/MGW 110 through RNC/nodeB (signaling 204 shown in FIG. 1 and ST204 shown in FIG. 2). Thus, Path C is established.
After establishment of Path C, MSC/MGW 110 exchanges signaling with P-GW/S-GW through MME (signaling 206 shown in FIG. 1 and ST206 shown in FIG. 2). Thus, Path A is deleted.
Hereinbefore, the operation of SRVCC handover has been described.
Further, as a technique that improves SRVCC to reduce the time necessary for switching data paths, there is an SRVCC method (eSRVCC: enhanced-SRVCC) that uses access transfer control function (ATCF) enhancement, as disclosed in NPL 3. An example of an operation of eSRVCC will be described with reference to FIGS. 3 and 4.
FIG. 3 shows a part of a configuration of a 3GPP mobile communication network that enables eSRVCC. The mobile communication network shown in FIG. 3 includes e-UTRAN, e-nodeB, a PS network, a CS network, a base station subsystem of the CS network, and IMS, similarly to FIG. 1. Here, an access transfer control function (ATCF) and an access transfer gateway (ATGW), in addition to CSCF and SCC AS, are present in IMS. In FIGS. 3 and 4, ATCF and ATGW are represented as a single node (ATCF/ATGW 320), but may be provided as separate nodes.
In FIG. 3, UE 100 and UE 102 are initially connected to the PS network, respectively (here, a wireless access network, a base station and the PS network on the side of UE 102 are not shown). That is, it is assumed that a VoLTE call is performed between UE 100 and UE 102. Here, it is assumed that UE 100 is handed over to the CS network during a call.
Path A, Path B, Path C and Path D indicated by solid lines in FIG. 3 represent paths through which speech data passes. Further, reference numerals 300, 302, 304 and 306 indicated by dashed lines in FIG. 3 represent paths through which signals in an eSRVCC handover process pass.
FIG. 4 is a sequence chart illustrating an operation of eSRVCC handover. UE 100 and UE 102 are each connected to the PS network (e-UTRAN), initially. In a system in which the eSRVCC handover is realized, in ATCF/ATGW 320, ATCF anchors signaling of IMS (IMS signaling), and ATGW anchors the speech data. That is, when a call between UE 100 and UE 102 starts, the IMS signaling for the call start is relayed by ATCF, and in a case where ATCF determines that anchoring of the speech data in ATGW is necessary, ATGW is allocated as an anchor point of the speech data. Thus, the speech data between UE 100 and UE 102 is transmitted and received through Path A and Path B.
If UE 100 is distant from a cover area of e-UTRAN, e-nodeB detects the fact, and exchanges signaling with RNC/nodeB through MME and MSC/MGW 110 (signaling 300 shown in FIG. 3 and ST300 shown in FIG. 4). In ST300, a data path in the CS network is prepared between nodeB and MSC/MGW 110. If the preparation is finished, a command for handover to UTRAN (CS network) is given to UE 100 from MME through e-nodeB.
Simultaneously with the process of ST300, MSC/MGW 110 transmits signaling to ATCF. Thus, a command for path switching is given to ATGW from ATCF, and a transmission/reception destination of speech data of ATGW is switched from UE 100 to MSC/MGW 100 (signaling 302 shown in FIG. 3 and ST302 shown in FIG. 4). That is, Path C is established. Further, if the path switching process to ATGW is finished, ATCF transmits indication signaling to SCC-AS (signaling 302 shown in FIG. 3 and ST302 shown in FIG. 4).
After handover to UTRAN, UE 100 exchanges signaling with MSC/MGW 110 through RNC/nodeB (signaling 304 shown in FIG. 3 and ST304 shown in FIG. 4). Thus, Path D is established.
After establishment of Path D, MSC/MGW 110 exchanges signaling with P-GW/S-GW through MME (signaling 306 shown in FIG. 3 and ST306 shown in FIG. 4). Thus, Path B is deleted.
Hereinbefore, the operation of eSRVCC handover has been described.
As a voice codec used in the CS network, an adaptive multi-rate wideband (AMR-WB) codec that is a wideband (WB) codec is widely used. AMR-WB is usable in a packet exchanging technique, and thus, may also be considered to be used in the PS network (VoLTE).
There is also a codec that supports an AMR-WB compatible mode as another codec other than AMR-WB used in the PS network (VoLTE) like enhanced voice service (EVS) described, for example, in NPL 4. The AMR-WB compatible mode assumes to be used as an AMR-WB codec with a legacy terminal that normally supports an AMR-WB codec. Therefore, when the codec is used in the PS network (VoLTE), an RTP payload format of the AMR-WB codec described in NPL 2 may be used.
In the related art, the narrowband (NB) codec is a codec that performs coding and decoding processing on a digital acoustic signal sampled at 8 kHz. The narrowband codec generally has a frequency band of 300 Hz to 3.4 kHz, but the frequency band is not limited to this, and can be within a range of 0 to 4 kHz. On the other hand, the wideband codec is a codec that performs coding and decoding processing on a digital acoustic signal sampled at 16 kHz. The wideband codec generally has a frequency hand of 50 Hz to 7 kHz, but the frequency band is not limited to this and can be within a range of 0 to 8 kHz. A super wideband (SWB) codec is a codec that performs coding and decoding processing on a digital acoustic signal sampled at 32 kHz. The super wideband codec generally has a frequency band of 50 Hz to 14 kHz, but the frequency band is not limited to this and can be within a range of 0 to 16 kHz.